Organic electroluminescent displays are becoming more important in various electronic products because they have advantages of thinness, low weight, low power consumption, easy manufacturing, and the capability of being formed on flexible substrates, thereby making them widely applicable. Organic electroluminescent device is generally composed of an anode electrode, a cathode electrode, and an organic light-emitting structure positioned between the anode and the cathode electrode, wherein the organic light-emitting structure may include hole-injection layers, hole transporting layers, organic light-emitting layers, electron transporting layers, electron-injection layers, and the like. Currently, methods for forming each layer of the organic light-emitting structure comprise: dry processes such as vacuum coating, organic vapor deposition and laser induced thermal imaging, etc., and wet processes such as spin coating, photolithography, and print patterning, etc., wherein it is preferable for forming the organic light-emitting layer by print patterning techniques in the wet processes due to its suitability for producing large size color displays. Print patterning technique can further reduce production costs by use of moving plates and printing equipment controlled by computers, wherein solution are patterned on predetermined regions according to requirements, obtaining the best material efficiency.
However, liquid is inclined to flow, and therefore it is not easy to form film layers of predetermined patterns for each pixel. To solve this problem, EP 0989,778A1 provides a positioned patterning method, as shown in FIG. 1. Banks 62 of polyimide are previously formed on the substrate 61 by photolithographic processes, and the surfaces of the banks 62 are treated with oxygen plasma and tetrafluoromethane (CF4) plasma so as to form fluoro-repelling film 63 on the surfaces of the banks 62. Next, hole-injection layer 64 is formed in the regions defined by banks 62. Then, organic light-emitting materials 65, 66 and 67 for producing various colors of light are separately inkjet printed on the hole-injection layer 64 and in the banks 62 with inkjet printing equipment 68. Due to the fluoro-repelling film on the surface of banks 62, organic light-emitting materials 65, 66 and 67 can be stably positioned in the regions defined by banks 62. Although organic light-emitting material can be properly positioned in the predetermined regions by the bank structures and surface treatment with repelling films, the process reduces production efficiency and increases production costs because of these additional procedures. Furthermore, it will result in electrical current leakage because of film defects between the cathode and anode electrodes, if the print patterning equipment misses discharging ink on desired area.
For this reason, U.S. patent application publication 20030157244 discloses a patterning method for organic light-emitting material positioned without making banks. As shown in FIG. 2, first, a conductive electrode layer 72 is formed on a substrate 71. Then, an insulating layer 73 is formed on the electrode layer 72 by spin coating. Next, a solution containing light-emitting conjugated polymer is deposited on the insulating layer by a microdroplet patterning method, wherein solvent in the solution can also dissolve material constituting the insulating layer 73. By this method, the solution being patterned on the insulating layer 73 can also dissolve the insulating material underlying the solution and form solution 74 containing light-emitting conjugated polymer and insulating material. Finally, solvent in solution 74 is removed to form the organic light-emitting layer. Due to the property of the solution dissolving the insulating layer 73, the method can position organic light-emitting material at predetermined regions. However, in order to drive the organic light-emitting device, the organic light-emitting material must extend through the insulating layer 73 to contact the electrode layer 72, but the area of contact narrows at depth and the presence of more insulating material will reduce the conductivity of the organic light-emitting layer, thereby resulting in an increase in the operating voltage for the emitting elements.
Moreover, it is usually to use active-matrix technique in color organic electroluminescent display devices for good image quality and especially moving images. Present active-matrix techniques mainly include using low temperature polycrystal silicon thin film transistors or amorphous silicon thin film transistors as pixel switching elements. There are several problems in practice in using low temperature polycrystal silicon thin film transistors as pixel switching elements. Therefore, in the liquid crystal display field, use of amorphous silicon thin film transistors as pixel switching elements is widely employed, and the transistor elements thereof have better uniformity and the technique thereof is relatively well developed. In particular, because n-type amorphous silicon thin film transistors have better element properties, it is desirable to apply n-type amorphous silicon thin film transistors to color organic luminescent display devices.
Thus, it is would be particularly desirable to have a method to position organic light-emitting material in predetermined regions by print patterning without additional structures such as bank structures, repelling films, and the like, and employ n-type amorphous silicon thin film transistors as pixel switching elements in color organic luminescent displays.